1. Field of the Invention
The present invention relates to voltage regulator circuits, and more particularly to digital pulse width modulation controller having a plurality of preset filter coefficients in order to accommodate various VIN/VOUT and load configurations.
2. Description of Related Art
Switched mode voltage regulators are known in the art to convert an available direct current (DC) level voltage to another DC level voltage. A switched mode voltage regulator provides a regulated DC output voltage to a load by selectively storing energy in an output inductor coupled to the load by switching the flow of current into the output inductor. A buck converter is one particular type of switched mode voltage regulator that includes two power switches that are typically provided by MOSFET transistors. A filter capacitor coupled in parallel with the load reduces ripple of the output current. A pulse width modulation (PWM) control circuit is used to control the gating of the power switches in an alternating manner to control the flow of current in the output inductor. The PWM control circuit uses feedback signals reflecting the output voltage and/or current level to adjust the duty cycle applied to the power switches in response to changing load conditions.
Conventional PWM control circuits are constructed using analog circuit components, such as operational amplifiers, and other discrete components. An error amplifier produces a voltage error signal corresponding to the difference between the feedback signals and a voltage reference. The voltage error signal controls a PWM modulator that determines the duty cycle applied to the power switches. Various resistors and capacitors are required to shape the transfer function of the error amplifier and thereby maintain stability of the feedback loop. Several factors can affect the feedback loop gain (e.g., changes in the input voltage, additional output capacitance, changing from a resistive to a constant current load, etc.) and need to be considered when selecting the resistors and capacitors. For this reason, the resistors and capacitors are typically not integrated into the PWM control circuitry, but rather are maintained as external components that can be selected to achieve a particular error amplifier transfer function.
A drawback of the conventional PWM control circuit is that there are quite a few discrete components that must be selected very carefully. High quality components having tight tolerances must be selected for this purpose. For example, the components must remain stable with changes in temperature in order to avoid loop gain changes and instability. The physical locations of the components is also critical to prevent pick up of noise that can affect the feedback loop. Another drawback with conventional PWM control circuits is that the hardwiring of the resistors and capacitors to the PWM control circuitry renders it very difficult to change the error amplifier transfer function. It would be desirable to have a PWM control circuit that can be operated using several alternative transfer functions to accommodate different operating conditions.
More recently, it is known to use digital circuitry in the PWM control circuit instead of the analog circuit components since digital circuitry takes up less physical space and draws less power. A conventional digital control circuit includes an analog-to-digital converter (ADC) that produces a digital error signal. The digital error signal is provided to a digital controller having a transfer function H(z) and shapes the transfer function H(z) to guarantee stability of the voltage regulator feedback loop with enough phase margin. The digital output of the controller is provided to a digital pulse width modulator (DPWM) that converts the output into a proportional pulse width signal that is used to control the power switches of the voltage regulator. Nevertheless, as with the analog PWM control circuit, the known digital PWM control circuits are only programmed for a single transfer function.
Thus, it would be advantageous to provide a digital pulse width modulation controller having a plurality of preset filter coefficients to accommodate different operating conditions of a switched mode voltage regulator.